Conventionally, rubber containing α,β-ethylenically unsaturated nitrile monomer units and conjugated diene monomer units or olefin monomer units (nitrile copolymer rubber) has been known as rubber with superior oil resistance. Its cross-linked products are mainly used as fuel hoses, gaskets, packing, oil seals, and other various types of rubber products used around oil for automotive applications.
Recently, due to the increasing global activities for protection of the environment, efforts have been made to reduce the amount of evaporation of gasoline and other fuel into the atmosphere. In fuel hoses, seals, packing, and other applications, further lower gasoline permeability is required. Further, fuel hoses are required to have resistance to the free radicals produced in sour gasoline (sour gasoline resistance).
Under these circumstances, Patent Document 1 proposes to stir and mix various types of rubber latexes, montmorillonite suspensions, and montmorillonite dispersants (pyrophosphoric acid compounds etc.) at a high speed to disperse and mix clayey materials in the rubber latexes to improve the properties of the cross-linked products. However, with this method, the gas barrier property of the obtained cross-linked products is improved, but the gasoline permeability is not lowered.
Further, with just blending in flat particle clay promising the effect of blocking permeation of gasoline into extremely high nitrile rubber (in general, meaning nitrile rubber with an amount of nitrile of 43 to 50 wt %), since this clay lacks affinity with extremely high nitrile rubber and has difficulty in dispersing, the gasoline permeability and sour gasoline resistance are not sufficiently improved. Note that, in a nitrile rubber, if the amount of nitrile is high, the gasoline permeability is improved.
Furthermore, by treating clay to make it organic treatment by an organic onium compound or by another lipophilic treatment, the dispersability is improved, but while an extremely high nitrile rubber cross-linked product including such a lipophilically treated clay exhibits the effects of increased tensile strength and elongation and reduced compression set, the absorption of gasoline is increased and the gasoline permeability becomes large.
For this reason, a rubber cross-linked product with an extremely low gasoline permeability and superior sour gasoline resistance has been demanded.
Patent Document 1: Japanese Patent Publication (A) No. 2006-70137